In a cellular communications system, the area over which service is provided is divided into a number of smaller areas called cells. Typically each cell is served from a base transceiver station (BTS) which has an antenna or antennas for transmission to and reception from a large number of user stations, normally mobile stations, e.g. mobile telephones. Examples of established harmonised cellular radio communications systems are Global System for Mobile Telecommunication (GSM) and General Packet Radio Service (GPRS) systems. A further harmonised standard being introduced is the Universal Mobile Telecommunications System (UMTS).
In cellular communication systems, multiple frequency channels are used in each cell, as well as used in different cells. A technique known as frequency hopping is used within cells to provide diversity. The choice of which frequencies are to be used in which cells plays an important role in the efficiency and quality of service in a system. For example, adjoining cells are prone to interfere with each other, so appropriate selection of frequencies may reduce such interference.
However, in order to achieve efficient usage of resources and provide service to large numbers of users, practically it is necessary to employ frequency plans that may involve potential interference. Various methods are known for testing systems to acquire data to enable frequency plans to be improved. One example is drive testing. Another example is to collect data from measurement reports that are anyway produced in the operation of the cellular communication system. Such measurement reports are produced by mobile stations and transmitted to control units of the system infrastructure.
The measurement reports comprise details of e.g. signal strength measured by the mobile station on the frequency or frequencies it is communicating with in its own cell. In addition, the measurement reports comprise similar details of frequencies that correspond to certain neighbouring cells. The mobile station is informed of which frequencies (and in effect which neighbouring cells) to measure and report by virtue of these being on a so-called neighbour list, determined by a control unit of the infrastructure of the system and communicated to the user station. The neighbour list is primarily provided to enable preparation for handover; however of interest here is the fact that measurements of the neighbouring cells and frequencies can be used in frequency planning.
In GSM systems, the neighbour list is transmitted to the mobile stations on a control channel called the Slow Associated Control Channel (SACCH), and hence the neighbour list may conveniently be called a SACCH neighbour list. Neighbours included for the primary purpose of potential handover planning are known as configured neighbours. For the purpose of providing measurement reports for collecting data for frequency planning, extra measurement frequencies are added to the SACCH neighbour list. The extra frequencies are known as test frequencies or test neighbours.
Conventionally, the same SACCH neighbour list is transmitted to all user stations in a given cell. Also, conventionally, the test frequencies employed in a given cell are changed once a day, so that over the course of a number of days all the frequencies it is desired to test may be included.
In each system there is a predetermined limit as to the total number of neighbour cells/frequencies that may be set. In GSM this limit is 32. However, there may be a large number of configured neighbours in a given cell, leaving only a small number of test frequencies that may be employed each day. Thus, the conventional approach is often slow and inefficient.
Moreover, by requiring different test frequencies to be tested on different days, the resulting data may not be properly representative of the real life situation, as statistical affects due to different communications traffic on different days may distort the results.